1. Field of Endeavor
The invention relates to a method for operating a fuel supply for a heat engine.
2. Brief Description of the Related Art
Heat engines, with gas turbine plants and/or steam turbine plants being first and foremost, which are used in most cases for power generation, are subject to strict safety guidelines which are outlined in normative regulations. In this context, reference may be made to the operating standard of the ISO Standard, ISO 21789, in which requirements for the safety of gas turbine plants, which are operated with liquid or gaseous fuel, are laid down. Of particular relevance are the specific requirements in Chapter 5.10 of the previously referred to ISO Standard for an operationally reliable fuel supply for the operation of a burner arrangement, under high working pressure, in which highly compressed combustion air and liquid and/or gaseous fuel are caused to be mixed to form an ignitable fuel-air mixture. In modern gas turbine plants, working pressures of more than 30 bar occur within the combustion process, which necessitate special provisions for a reliable fuel feed into the combustion process. Particularly in cases in which the fuel feed has to be interrupted in the short term, or a changeover from one type of fuel to the other (liquid to gaseous, or vice versa) is carried out, it is necessary to ensure that no uncontrolled flashbacks along the fuel feed line or other uncontrolled fuel explosions take place. For avoiding such irregular combustion phenomena, provision is made along the fuel feed line for a specially designed safety valve system—a so-called double block and vent valve system —which ensures a safe interruption of the fuel both in cases of a controlled shutdown of the gas turbine plant and in emergency cases. The construction and the mode of operation of such a double block and vent valve is described in detail in Chapter 5.10.5 in the previously cited ISO Standard, ISO 21789.
In the previously current practice for executing a rapid shutdown of the combustion process 3, the feed of fuel is prevented as a result of simultaneous closing of a block valve and control valve, as a result of which a proportion of the fuel, under working pressure conditions of the combustion process, is trapped inside a line section of the fuel line and vent line, which section is delimited on one side by the block valve, control valve and vent valve which are closed in each case. Furthermore, it may be assumed that this line section traps a fuel volume of V1.
After closing of the block valve and control valve has taken place, the vent valve is opened, as a result of which the proportion of pressurized fuel is drained via the vent valve into the region of the drainage point. If the fuel is a gaseous fuel, for example, then this can escape via a type of chimney at a safe place into the open atmosphere.
In this way, a safe disposal of the proportion of fuel inside the line section which delimits the fuel volume V1 is certainly ensured, but a substantial additional volumetric proportion V2 of fuel remains in the fuel line which extends downstream of the control valve and upstream of the combustion process. This proportion of residual fuel can lead to unwanted ignition or combustion reactions which, especially in the case of fuel flashbacks along the fuel line, can lead to significant damage. It is now necessary to reliably exclude these uncontrolled combustion events. The current measures for avoiding such uncontrolled combustion events provide purging of the fuel line which leads to the combustion process, during which, with the control valve open, purging media, such as CO2, N2, air or the like, is fed via additional purging lines into the fuel line which leads to the combustion process. However, such purging measures can contribute or lead to further malfunctions of the combustion process, as a result of which at least the proportion of residual fuel in the region of the burner arrangement is caused to combust.